1. Field of the Invention
This invention relates to a cell in which a roll electrode element is accommodated, and more particularly to the roll electrode element produced by winding an electrode laminate around a core having an approximately rhombic shape in section, in which the respective electrodes of the electrode element are effectively prevented from being dislocated upon winding whereby fluctuation of the discharge capacity of the cell is considerably prohibited.
2. Prior Art
Recently, accompanying with a remarkable progress of the electronics techniques, reduction in size and weight of electronic equipments has been realized successively. Under these circumstances, there is an increased demand that a cell serving as a power source of the portable electronic equipments also has a reduced size, a reduced weight and a high energy density.
Hitherto, a secondary cell generally used has been predominantly of an aqueous electrolyte type such as a lead battery, nickel-cadmium battery, or the like. However, these cells of such an aqueous electrolyte type are unsatisfactory with respect to a degree of reduction of its weight and increase in its energy density though it shows a high cycle characteristic.
On the other hand, intense study and investigation have been recently made to develop a non-aqueous electrolyte type secondary cell in which a material capable of dope or undope of a lithium ion, such as lithium, a lithium alloy or a carbonaceous material is used as a negative electrode and a lithium-containing composite oxide such as lithium-cobalt composite oxide is used as a positive electrode. Since the non-aqueous electrolyte type secondary cell advantageously has not only a high energy density but also a low self-discharge and a reduced weight, an increased attention has been paid to the production of such a cell.
Meanwhile, in the field of the electronic equipments mentioned above, in association with continuous reduction in size and weight thereof, it is required to achieve a rectangular box-shaped cell having a thin thickness from a standpoint of saving a space in the electronic equipment.
In the case of such a rectangular box-shaped cell, there has been generally used a laminate-type electrode element composed of a plurality of strip-like positive electrodes, a plurality of strip-like negative electrodes and separator films interposed between the positive and negative electrodes, which are arranged alternately in an overlapped relation to each other. However, it is difficult to optimize a thickness of such a laminate-type electrode element.
For example, if each of the strip-like positive and negative electrodes has a relatively large thickness, the number of the strip-like positive and negative electrodes used for the production of the laminate-type electrode element must be limited to permit the laminate-type, electrode to be received within a casing of the cell. In this case, although it is easy to handle the laminate-type electrode element due to its simplified construction, the laminate-type electrode element has a small surface area serving for an electrode reaction between the positive and negative electrodes so that its heavy-load characteristic becomes unsatisfactory and rapid charging is not applicable thereto.
On the other hand, if the strip-like positive and negative electrodes have a small thickness, the number of the strip-like positive and negative electrodes used for the production of the laminate-type electrode element can be increased. However, in this case, since a construction of the laminate-type electrode element becomes complicated so that it is difficult to handle the laminate-type electrode element.
Under these circumstances, there has been proposed a roll electrode element composed of an elongated sheet-like positive electrode and an elongated sheet-like negative electrode and separator films. The sheet-like positive electrode, the sheet-like negative electrode and the separator films are arranged alternately in an overlapped relation to each other to form a electrode laminate which is then wound into a roll to obtain a roll electrode element. Such a roll electrode element has a multi-layered structure when viewed in cross-section thereof so that a large surface area for an electrode reaction between the positive electrode and the negative electrode is assured. In addition, only one sheet of each of the positive and negative electrodes is required to produce the roll electrode element, which achieves an considerably simplified construction of the electrode element.
However, if it is intended to apply such a roll electrode element to a rectangular box-shaped cell, the following problems are caused. That is, the roll electrode element is generally produced by winding the above-mentioned electrode laminate around a core having a circular or ellipsoidal shape in section to form a multi-layered roll. After completion of the winding around the core, the core is removed from the roll electrode element. Thereafter, the roll electrode element is required to be compressed in the diametrical direction thereof to thereby deform it into an ellipsoidal shape in section, such that the roll electrode element can be accommodated within a casing of the rectangular box-shaped cell.
However, in the case where the core having a circular or ellipsoidal shape in section is used for forming the roll electrode element, looseness is caused between the respective electrodes, particularly at inner peripheral portions in the proximity of folded portions of the roll, so that a clearance is created between the positive and negative electrodes. As a result, a uniform electrode reaction between the positive and negative electrodes cannot be maintained, which leads to undesirable deterioration or fluctuation of the discharge capacity of the cell.
In order to overcome the above-mentioned deficiencies, several proposals have been conventionally made. For example, Japanese patent laid-open publication No. 98,801/94 discloses an electrode element which is produced by winding an electrode laminate around a core having a flat-plate shape. However, since the flat plate-shaped core has a unsatisfactory mechanical strength (hardness), an undesirable dislocation is caused between the positive and negative electrodes upon winding. Namely, the sheet-like negative electrode is projected from the sheet-like positive electrode in the direction of the width thereof so that a portion of the positive electrode remains uncovered with the negative electrode. Such an uncovered portion generated in the roll electrode element, causes deterioration of the discharge capacity of the cell, as disclosed in Japanese utility model laid-open publication No. 150,760/90.